The feeding and/or weighing of solids and liquids is a task encountered in a myriad of industrial processes, for example, the production of foods, chemicals, plastics, explosives, pharmaceuticals, and a wide array of other products. Often in such applications, the rate at which material is fed from a supply hopper must be accurately controlled. In other applications, the flow of material is performed in an unregulated, uncontrolled fashion, known in the trade as "wild-flow." A "wild-flow" weigher is one in which a continuous supply of materials is delivered continuously, but in an unregulated fashion.
In those applications where controlled feeding or weighing is desired, two fundamental types of continuous metering mechanisms exist: volumetric feeders and gravimetric feeders. Volumetric feeders dispense material by volume and without use of feedback control. Feeders which do monitor their own feeding performance and automatically correct for variations from the desired performance are called "gravimetric" or weigh feeders. Several types of gravimetric feeders exist, the earliest to have been developed being the weigh-belt and weigh-auger. Weigh-belt and weigh-auger feeders measure the weight of material on a belt or in an auger during operation, that is, while it is feeding material, and compare that weight to an expected or "set" weight. In the case of a weigh-belt, the scale measures the weight of the belt or a section of the belt loaded with material and compares it with the expected or "set" weight. If the measured weight of the belt plus the material on the belt is more or less than the expected (or set) weight, a command is generated calling for more or less material per unit time to be fed onto the belt from the feed source. Exemplary weigh-belt and weigh-auger feeders are disclosed in U.S. Pat. Nos. 3,494,507 and 3,710,983 assigned to the assignee of the present invention.
A common problem with weigh-belt and weigh-auger feeders is material accumulating on the belt, auger or other critical components of the weighing mechanism and thereby changing the scale's "zero reference" and thus, the weight being monitored. If material is adhering to the belt auger or other scale components, the weight measured will be higher than appropriate for the amount of material actually being fed because the scale's "zero" has increased improperly. The problem is even worse if this accumulation builds up and falls off the belt or auger intermittently, thereby causing the "zero" weight of the belt or auger to fluctuate. In short, this type of gravimetric system, while using feedback principles to offer some control, has substantial limitations.
The most accurate form of gravimetric feeding is the loss-in-weight system, in which the gradual decrease in weight measured by a scale caused by the constant release of material from the scale mounted container (hopper or tank) is monitored and compared to an expected decrease in weight to assess performance. Loss-in-weight feeders are not in any way affected by the build-up of material, whether continuous or intermittent, because they monitor not absolute weight, but the change of weight over time. Exemplary loss-in-weight feeders, which include control means for protecting such systems even against extraneous disturbances of the type which long plagued the weight-loss weigh feeding industry, are shown in U.S. Re. Pat. Nos. 30,967, 32,101 and 32,102 and U.S. patent appl. Ser. No. 889,471, all assigned to the assignee of the present invention.
The present invention combines a wild-flow stream of material and loss-in-weight weighing to produce a system having unique accuracy-producing weighing characteristics.